The present invention relates to color printing method used for color-printing a full-colored image. The image is picked up by an image pickup device such as a color camera, image scanner, or the like. Thereafter the image is processed in an image processor. In a preferred embodiment the invention is directed to a color printing method which makes it possible to output a high contrast and good quality image from an output device, such as a color inkjet printer, heat transfer color printer of a melting type, or the like.
In general color printing, as shown in FIG. 21, uses a full-colored image picked up by a pickup device 1 such as a color camera, image scanner, or the like which is processed by an image processor 2 and a processing result obtained is outputted as a full-colored image by a color printer 3. Some color printers use three different color inks of yellow, magenta and cyan, and other color printers use black ink in addition to those inks. It cannot be said, however, that color of each of such inks has an ideal spectral distribution, because of the following limitations derived from printers' properties.
(1) In the case of the color inkjet printer, it is necessary to use inks of low viscosity in order to avoid clogging of the ink heads, and to select to ink materials that permeate printing paper to an appropriate depth of the paper and to form ink dots of a proper diameter.
(2) In the case of the heat transfer color printer of ink melting type, because inks are heated by a printing head of a very small area and transferred onto printing paper, it is necessary to select ink materials which has the necessary melting points, response times, adhesivity to printing paper, etc.
Because of such limitations, it often happens that when a natural image having several thousands of gradated colors, i.e., a full-colored image, is printed, an output image has black with a low density. The low density of the black deteriorates contrast and quality of the image printed.
Color printers that output inks onto printing paper dot by dot can be classified as follows:
(a) Ink density for each dot is changeable.
(b) Ink density for each dot is fixed (to binary digit 0 or 1).
In the case of outputting a full-colored image using a color printer adopting the principle (b) above, a displaying method based on a pseudo-half tone represented by a dither matrix system is adopted. This method expresses density information of each of picture elements composing an image using density of dots per unit area. FIG. 22 shows a dither matrix of Bayer type by way of example. This matrix table is composed of 4 rows by 4 columns and is designed so that ink dots are dispersed as much as possible. Color density of an ink color for one picture element has sixteen levels (0-15). Numerals 0-15 shown in FIG. 22 are thresholds to decide the arrangement of ink dots to be printed out. Value of image information is compared with the threshold, and the ink dot is printed out or not according to a result of the comparison.
Generally, a printer head of a printer using four different color inks has ink heads 4 for outputting yellow, magenta, cyan and black inks aligned in a lateral or longitudinal direction, as shown in FIG. 23. These ink heads 4 are moved in the directions X and Y relatively to printing paper. A pitch of ink dots is equal to a minimum moving pitch .DELTA.P of the ink heads 4 in the directions X and Y. Accordingly, double-output of inks of different colors at a common position is possible, but a double-output of the same ink is impossible.
In order to overcome the problem of the low density of black ink, there is a method in which, using the printer head to output the black, yellow, magenta and cyan inks, all the inks are outputted at the same place. This method, however, if applied to a color inkjet printer, causes the printer head to output ink of a quantity beyond the water absorptive power of a printing paper, which causes formation of dots of an unacceptable diameter, blur, contact of the ink outputted and the printer head causing stains on the paper.
There is a further method in which black ink and either one of yellow, magenta and cyan inks are outputted at the same place. This method seeks avoiding blurring and staining on printing paper. But, if black ink is always combined with yellow ink, for example, an output image will have a yellowish hue. On the other hand, in combination with cyan ink, the image will have a light bluish hue, and with magenta ink, the image will be purplish.
According to a still further method, yellow, magenta and cyan inks are outputted on black ink dots in turns in order to avoid the problem of a printout image having a hue of a specified color. This method has a problem, however, that in some cases cyclic outputting of yellow, magenta, and cyan inks causes stripes on an output or printout image, deteriorating quality of the image to a great extent.